72 



POPULAR HISTORY OP LICHENS. 



stance of the thallus, and opening on its surface by very 

 delicate pores. Sometimes they are superficial, and seated 

 on the extremities of the thalline segments, or of various 

 appendages, as in Cetraria and Cladonia. The cavity of 

 the spermogone is usually simple and rounded; sometimes 

 it is divided into two or more sinuous cavities or cells, in 

 which case its form becomes more elongated and irregular. 

 Its outer wall or envelope is generally dense and coloured, 

 frequently dark brown or black, especially towards the apex 

 of the organ, or that portion of it which is visible on the 

 surface of the thallus ; it is then easily distinguishable, and 

 may be separated in the mature state from the surrounding 

 thalline tissues. Sometimes however the walls are pale-co- 

 loured, and more or less adherent to, or incorporated with, 

 adjacent textures. Its inner wall is composed of a series of 

 delicate, simple or branched, one-celled or jointed filaments, 

 which, from their function as the generators or supports of 

 the spermatia, are denominated Sterigmata (crTtfpiyfjLa, a 

 support) ; these are arranged vertically to the outer wall, and 

 project in a convergent manner into the cavity of the sper- 

 mogone. They are very hygrometric, imbibing water with 

 the greatest rapidity; and they are bathed in a copious 

 transparent gelatine, which fills more or less the whole 



